Laboratory of Plant Molecular Physiology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo 113-8657, Japan.
J Biol Chem. 2010 Jan 8;285(2):1138-46. doi: 10.1074/jbc.M109.054288. Epub 2009 Nov 9.
Sugars play indispensable roles in biological reactions and are distributed into various tissues or organelles via transporters in plants. Under abiotic stress conditions, plants accumulate sugars as a means to increase stress tolerance. Here, we report an abiotic stress-inducible transporter for monosaccharides from Arabidopsis thaliana that is termed ESL1 (ERD six-like 1). Expression of ESL1 was induced under drought and high salinity conditions and with exogenous application of abscisic acid. Promoter analyses using beta-glucuronidase and green fluorescent protein reporters revealed that ESL1 is mainly expressed in pericycle and xylem parenchyma cells. The fluorescence of ESL1-green fluorescent protein-fused protein was detected at tonoplast in transgenic Arabidopsis plants and tobacco BY-2 cells. Furthermore, alanine-scanning mutagenesis revealed that an N-terminal LXXXLL motif in ESL1 was essential for its localization at the tonoplast. Transgenic BY-2 cells expressing mutated ESL1, which was localized at the plasma membrane, showed an uptake ability for monosaccharides. Moreover, the value of K(m) for glucose uptake activity of mutated ESL1 in the transgenic BY-2 cells was extraordinarily high, and the transport activity was independent from a proton gradient. These results indicate that ESL1 is a low affinity facilitated diffusion transporter. Finally, we detected that vacuolar invertase activity was increased under abiotic stress conditions, and the expression patterns of vacuolar invertase genes were similar to that of ESL1. Under abiotic stress conditions, ESL1 might function coordinately with the vacuolar invertase to regulate osmotic pressure by affecting the accumulation of sugar in plant cells.
糖在生物反应中起着不可或缺的作用，并通过植物中的转运蛋白分布到各种组织或细胞器中。在非生物胁迫条件下，植物会积累糖以提高其对胁迫的耐受性。在这里，我们报道了一种来自拟南芥的非生物胁迫诱导的单糖转运蛋白，称为 ESL1（ERD six-like 1）。ESL1 的表达在干旱和高盐条件以及外源施加脱落酸时被诱导。使用β-葡萄糖醛酸酶和绿色荧光蛋白报告基因进行启动子分析表明，ESL1 主要在周韧皮部和木质部薄壁细胞中表达。在转基因拟南芥植物和烟草 BY-2 细胞中，检测到 ESL1-绿色荧光蛋白融合蛋白的荧光位于液泡膜上。此外，丙氨酸扫描诱变显示 ESL1 中的 N 端 LXXXLL 基序对于其在液泡膜上的定位是必需的。表达定位于质膜的突变 ESL1 的转基因 BY-2 细胞表现出对单糖的摄取能力。此外，突变 ESL1 在转基因 BY-2 细胞中对葡萄糖摄取活性的 K(m)值非常高，并且其转运活性与质子梯度无关。这些结果表明 ESL1 是一种低亲和力促进扩散转运蛋白。最后，我们检测到在非生物胁迫条件下液泡转化酶活性增加，并且液泡转化酶基因的表达模式与 ESL1 相似。在非生物胁迫条件下，ESL1 可能通过影响细胞内糖的积累，与液泡转化酶协同作用来调节渗透压。
